Frequency monitors may be employed to measure periodic signals, for example, electrical pulses from a tachometer or the like. Low-frequency signals are typically measuring the time between signal “features” of two successive cycles of the signal, for example, the rising edge of a square-wave pulse or zero crossing of a sine wave. The measured time may be obtained by counting “ticks” of a high-speed clock between the occurrence of these signal features and this count may be inverted to obtain a measure of frequency.
As the frequency rises, the number of measured clock ticks falls decreasing the effective resolution of the measurement. This decrease in resolution may be avoided by increasing the resolution of the clock (for example, using a higher clock frequency) or by converting to a second measurement method of counting the number of signal features for a given unit of time, for example, the number of rising edges or zero crossings per second. This second method provides increased resolution as the frequency rises and more clock ticks are counted in that given unit of time. This approach yields a direct measurement of frequency without the need for inversion.
In order to accommodate a wide variety of frequencies to be measured, frequency monitors may have manual settings allowing the user to set the clock frequency (effectively the resolution of the measurement) or to select between these two above-described methods of frequency measurement: (1) timing the duration between pulses or (2) counting a number of pulses for a given time duration.
This need to manually set the frequency monitor or to determine the likely frequency of the input signal can be problematic in an automatic control environment where knowledge about the frequency of the input signal may not be well known and there is a desire to avoid the need for human intervention.